SURVEY" OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1891. 555 
The lines given by these tables differ so little from those shown in Maps 1-3 for 
the 1891 Survey, that it is difficult to exhibit them in the same maps unless they are 
on a lar 2 :e scale. They are however drawn in continuous blue lines. 
Local and Regional Disturbances. 
Having thus proved that the two Surveys are in close accord as to the form and 
direction of the terrestrial isomagnetics, we have prepared the following tables, in 
which the observed values of the elements reduced to January 1, 1891, the calculated 
values for the same dates, and the difference between the observed and calculated 
values, i.e., the Disturbances, are sliown. 
The stations, whether belonging to the earlier or later survey, are numbered 
consecutively, but in the second column the numbers assigned in our previous Memoir 
to first survey stations are also given. 
The observed values given in the “ 1890 Memoir” are reduced to January 1, 1891, 
as described above. Two corrections are applied to the calculated values, viz.: firstly, 
that for secular change ; and secondly, that for the difference between the 1886 
isomagnetics (corrected for secular change) and the mean isornagnetics of the two 
surveys. Hence the Disturbances for these stations are not the same as those given 
in the “ 1890 Memoir,” but, as has been shown, the differences are for the most 
part small. The calculated values may also be determined independently from 
Tables XX.-XXII., p. 544 above. 
Thoughout the Tables the last figure in the observed and calculated values of 
the elements must be considered as only approximate. The Declinations and Dips 
may be affected with errors of reduction amounting to one or two tenths of a minute 
of arc, and the Horizontal Force by one or two units in the last place. In con¬ 
sequence of these the Vertical Forces may be erroneous to the extent of several 
units in the last place. Such differences are in all cases very much less than the 
error of experiment, and the labour involved in ensuring the accuracy of tlie last 
place would not really have added to the trustworthiness of our conclusions. We 
give the last place in the Tables, however, because we believe that in the large 
majority of cases it is very nearly if not absolutely correct. 
The Disturbances of the Declinations, Horizontal Force, Dip and Vertical Force, are 
entered in Maps 9-12. 
When the observed is greater than the calculated value of the Element, the Disturb¬ 
ance is positive and is indicated by a red figure. Negative Disturbances are printed 
in blue. As the boundaries between the regions of positRe and of negative Disturb¬ 
ance are sufficiently indicated by the difference of colour, we have not drawn lines of 
separation between them. 
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